Amphibious hovercraft usually employ ducted air propellors for forward motion and directional control over land, water, snow and ice. Low speed thrust and lack of control in cross winds or when climbing or descending steep slopes are problems which have plagued the industry. Air propellor performance, in terms of thrust, efficiency and noise level, and installation, in terms of drivetrain, mounting, and freeboard, have presented serious constraints to hovercraft performance and fuel economy.
Hybrid amphibious vehicles utilizing an alternative form of propulsion, paddletrack propulsion, were extensively used during the second world war but were found to provide inadequate thrust and efficiency at speed over water. Hydrodynamic design considerations were over-ridden by the need for closely spaced cleats to ensure adequate ground tractive effort and support. These deficiencies are an inherent flaw in the propulsion principle as explained hereinbelow.
Paddletrack propulsion is one of the oldest forms of mechanical drive. In modern applications, the paddles, which extend at 90.degree. to the direction of travel, are mounted on endless movable tracks, similar to the treads or cleats of conventional tracked vehicles. About one third of the tracks act upon the water simultaneously.
In order to obtain a reasonable degree of efficiency in water with a propulsion system of this type, it is essential that undisturbed water be fed constantly into the spaces between the cleats if a high degree of frictional resistance or tractive effort between the water and the cleats is to be produced. While this can be achieved by providing relatively wide inter-cleat spacing, the resultant track does not present sufficient blade area to "bite" the water since only stub blades are practical for amphibious vehicles. High or deep blades would quickly become damaged on ground and also present extreme bending moments on the blade mounting to the belt or track. Thus, the track would be rendered incapable of adequately supporting the vehicle on land. Conversely, narrow inter-cleat spacing would cause the cleats to churn through the water which would greatly disturb the water between the cleats and result in an inefficient propulsion system.
The present invention was developed in an attempt to overcome the disadvantages of both types of propulsion arrangements discussed above. For reasons which will become clear later, the invention is considered inherently useful in a variety of other applications such as, for example, oil pollution cleanup, firepump suction in shallow water, ice prevention and removal, bolt-on thrusters for ship recovery, jetty (safety) thruster devices, material dewatering conveyors, underwater dredging, trenching and mining and crawler propulsion pads. Thus, the present invention is not to be regarded as being limited to vehicle propulsion systems.